Cyclic Compression-Flexion Loading of the Human Lumbar Spine

STUDY DESIGN: The present study was designed to investigate the biomechanical behavior of the lumbar spine under controlled complex physiologic situations with chronic input. OBJECTIVE: The objective was to determine the response of the human cadaver lumbar spinal column under repetitive compression-flexion forces. SUMMARY OF BACKGROUND DATA: Studies have been conducted in the past to determine the biomechanical response of the spine under uniaxial or pure forces. There is no methodology that can be used to apply and continuously quantify the fatigue response of the lumbar spinal column under controlled combined complex loading vectors (e.g., compression flexion). METHODS: Intact cadaver lumbar columns (L1-L5) were mounted with the superior end in contact with a ball-transfer mount, inducing a flexion load to the spine while allowing multiple degrees of freedom. The distal portion of the specimen was attached to a six-axis load cell to quantify the force sustained by the specimen during the entire loading cycle. The applied load and piston deformation and the generalized six-axis force histories were gathered as a function of time using a digital data acquisition system. RESULTS: The stiffness versus number of cycles (K-N) response exhibited nonlinear characteristics. The stiffness increased initially and then stabilized after 1,000-2,000 cycles of loading, delineating the viscoelastic characteristics of the spine. The initial stiffness increase before stabilization was found to be significantly different (P < 0.025) compared to the stiffness beyond 2,000 cycles. CONCLUSIONS: The data suggest that the fatigue response can be understood by cyclically loading the ligamentous lumbar spine preparation to approximately 2,000 cycles.